Arrangement in industrial trucks

ABSTRACT

An arrangement in industrial trucks of the kind which include a chassis (3) carried by support wheels (5, 6) which lack spring suspension, and driven by a sprung drive-wheel (7) arranged between two support wheels. A hydraulically operable load carrying device (2) is provided for lifting primarily goods-laden pallets. The sprung movement of the drive-wheel (7) relative to the chassis is effected with the aid of a spring assembly (22) which includes link arms (25, 26) and a spring (24) both being arranged between the chassis and the drive-wheel. The spring (24) includes a hydraulic working piston-cylinder device (27) by means of which the spring assembly can be given a desired spring bias or spring force. The piston-cylinder device is connected to the hydraulic system of the load carrying device in a manner to increase automatically the spring bias, and therewith the pressure exerted by the drive-wheel against the floor or like surface when the load carried increases.

This is a continuation of application Ser. No. 877,969 filed June 24,1986 now U.S. Pat. No. 4,750,579.

BACKGROUND OF THE INVENTION

The present invention relates to an industrial truck having anarrangement for increasing the braking force of at least one wheel ofthe industrial truck relative to a driving surface on which theindustrial truck is driven, the industrial truck comprising a chassiswhich is supported on wheels which have substantially no suspension, andwhich is driven and braked by a sprung drive-wheel arranged between twoof the support wheels and a load-carrying device which can be raised andlowered relative to the chassis by means of a hydraulic liftingarrangement intended therefor, and in which trucks there is arrangedbetween the chassis and the drive-wheel a spring assembly which isoperative in applying a spring force to the drive-wheel in a directiontowards the driving surface on which the truck stands.

Industrial trucks of this kind are used for transporting goods-ladenpallets within, for example, industrial premises. The trucks may beintended solely for low lifts, or for both low and high lifts, whenhandling stock in pallet racks or the like. Normally, the supportingwheels are completely without suspension, whereas the drive-wheel ispressed against the driving surface with a given spring force, in orderto hold the wheel in contact with the driving surface when meetingirregularities therein. The magnitude of this spring force is determinedby the condition that the chassis must rest on all support wheels, evenwhen being driven with no load, so that stability remains the same, i.e.the spring force must not be of such magnitude that the drive-wheellifts the support wheels from the driving surface. This means, however,that the surface engagement pressure of the drive-wheel remainsunaltered when switching from light to heavier loads, which results indisadvantageous skidding and sliding tendencies, and above allsubstantially lengthens the braking distance as a result of the increasein kinetic energy. Since trucks of this kind have become progressivelylighter in weight in relation to their load carrying capacity and can bedriven at high speeds, the aforementioned drawback has reached greaterproportions in recent years.

SUMMARY OF THE INVENTION

Consequently, the object of the invention is to provide in trucks of thekind described, an arrangement with which the frictional force betweendrive-wheel and driving surface, and therewith the braking force, can beincreased with increasing load weights on the truck. This object isachieved by providing means for increasing the spring force of thedrive-wheel with increasing load-weights carried by the truck, and morespecifically by providing means for increasing the spring bias of thespring means. Further objects of the invention and advantages affordedthereby will be evident from the following description. These furtherobjects and advantages are achieved and afforded by the characterizingfeatures set forth in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a truck constructed in accordance with theinvention;

FIG. 2 is a plan view of the truck illustrated in FIG. 1;

FIG. 3 is a side-view of the drive-wheel and its associated springassembly, incorporated in the truck illustrated in FIGS. 1 and 2;

FIG. 4 is a sectional view of part of the spring assembly illustrated inFIG. 3;

FIG. 5 illustrates a hydraulic circuit for a low-lift truck according toFIGS. 1 and 2; and

FIG. 6 illustrates a corresponding hydraulic circuit for a high-lifttruck.

DETAILED DESCRIPTION

The illustrated low-lift truck is of conventional construction andincorporates a battery and motor housing 1, and a load carrying device2, all supported on a common chassis 3. The truck is steered with theaid of a movable steering handle 4, and may optionally be provided witha driving cabin or similar location, so that the truck operator canaccompany the truck while sitting down or standing up. The truck issupported on two front and two back support-wheels 5 and 6 respectively,of which the front wheels have the form of conventional articulatedwheels. Arranged on the chassis, centrally between the articulatedwheels 5, is a drive-wheel 7 with which the truck is driven, braked, andsteered. The support wheels 5, 6 are substantially without any form ofspring suspension whatsoever, whereas the drive-wheel 7 is sprung inorder to enable it to follow irregularities, in the form of gulliesetc., in the driving surface or floor 8. The load carrying device 2suitably has the form of a fork-lift assembly which can be raised andlowered in a known manner with the aid of a hydraulic ram device 9.

The sprung suspension of the drive-wheel 7 is illustrated in FIG. 3,from which it will be seen that the drive-wheel 7 is firmly connected toa power assembly 10, which comprises an electric motor 11 located abovethe drive-wheel 7 and a gear unit 12 located in the hub 13. The powerassembly 10 is carried by a flat motor support 14 and is journalled inrelation to said support by means of a bearing 15. The electric motor 11is arranged vertically so that its rotation axis 16 will beperpendicular to the drive-wheel axis 17. The motor support 14 and thusthe power asembly 10 is suspended on the chassis 3 through a springmeans 22 incorporating first and second spring elements 23 and 24, andan upper and a lower link-arm pair 25 and 26 which extend parallel withone another between the chassis and the motor support. The first springelement 23 is attached between the chassis 3 and the motor support 14,while the second spring element 24 acts between the chassis and thelower link arms 26. Both spring elements 23, 24 preferably comprise twocoil springs. However, whereas the first spring element 23 is arrangedto exert a constant force against the driving or floor surface, thesecond spring element is provided with power means for increasing thespring force with increasing load-weight on the load carrying device 2.This means includes a hydraulic piston-cylinder device 27 whose piston28 lies against the chassis 3, while the cylinder 29 of said device liesagainst the upper end of the spring via a flange 30. The workingpiston-cylinder device is connected to the hydraulic system serving thelifting ram 9 and is fed with hydraulic fluid through an inlet 31. Thespring 24 extends around the cylinder 29 and is supported by a connectorplate 32 arranged between the lower link arms 26. When the hydraulicpressure increases, the cylinder 29 and associated flange are presseddownwards, therewith compressing the spring 24 and exerting a greaterspring force downwardly on the link-arm pair 26 and therewith on thedrive-wheel 7. The piston 28 is mounted in an aperture 33 in the chassis3 in a manner which enables the working cylinder to swing slightly whenthe link-arms are swung. The spring element is shown in the positiontaken thereby when no load is placed on the load carrying device 2. Inthis position, the spring force applied on the drive-wheel is adapted togive the wheel a sprung movement of plus or minus 2 mm without risk ofany of the support wheels 5 lifting from the floor surface. The link armsystem will make the drive-wheel suspension very stable without any riskfor binding or jamming. This is particularly important since thevertical movement of the wheel is very small and said movement has to becarried out in spite of strong horizontal forces which act on the wheelespecially when braking.

FIG. 5 illustrates schematically a hydraulic circuit for the aforesaidlow-lifting truck. As shown in the illustration, a pump 34 driveshydraulic fluid through a non-return valve 35 to the lifting ram 9,which communicates with the working piston-cylinder device 27 such thatthe device is subjected to the same hydraulic pressure as the liftingram. Since the pressure prevailing in the ram is totally dependent onthe weight to be carried, the bias or spring force of the spring 24 willalso be proportional to the weight of the load. The hydraulic circuitalso incorporates conventionally a lowering valve 36 and an overflowvalve 37 through which hydraulic fluid can be returned to a tank 38.Thus, the arrangement according to the invention is effective inincreasing automatically the pressure of the drive-wheel against thefloor or driving surface when the load increases. This increase in wheelpressure results in greater friction between the drive-wheel and thefloor surface, and therewith enables the braking distance to beshortened considerably in relation to braking distances when the wheelpressure remains unchanged.

FIG. 6 illustrates a modified hydraulic circuit intended for high-liftindustrial trucks. These trucks are designed for transporting loads toand from pallet stacks or like locations, and for lifting the loads inthe actual stack location. The hydraulic circuit incorporates a slidevalve 39 or the like, for controlling lifting and lowering movements. Acut-off valve 40 is provided for interrupting communication between theram 9 and the piston-cylinder device 27 when the load carrying device 2has reached a given height. This switch-over is effected with the aid ofschematically illustrated control means 41, which may comprise, forexample, an electric switch and transducer means located on the liftingarrangement. The valve 40 is preferably adapted to take the aforesaidblocking mode when a lifting height of about one meter is reached,whereupon the piston-cylinder 27 is without pressure and is connected tothe tank 38. This system enables a full spring force to be exerted onthe drive-wheel when the truck is driven with limited raised loads, andfurther enables the spring force to be reduced when driving loads whichare lifted to higher levels, so that the wheel pressure has nodeleterious effect on the stability of the truck. This enables the truckto be driven at high speeds to and from the pallet locations whilemaintaining the desired braking effects. It is always necessary forsafety reasons to drive slowly in the actual vicinity of the palletlocations, therewith rendering additional braking force unnecessary,i.e. disconnection of the piston-cylinder device has no real detrimentaleffect.

It will be understood that the invention is not restricted to theillustrated and described embodiment, and that various modifications canbe made within the scope of the following claims.

We claim:
 1. In an industrial truck, an arrangement for increasing thebraking force of at least one wheel of the industrial truck relative toa driving surface on which the industrial truck is driven, theindustrial truck including a chassis carried on support wheels whichhave substantially no spring suspension and which are driven and brakedby a sprung drive-wheel arranged between two of the support wheels and aload carrying device which can be raised and lowered relative to thechassis by means of a hydraulic lifting arrangement, and which truckfurther includes a spring means arranged between the chassis and thesprung drive-wheel and which is adapted to exert on said drive-wheel aspring force acting towards said driving surface,the improvement whereinsaid spring means includes: coupling means coupling said chassis to saiddrive-wheel while permitting substantially vertical movement of saidchassis relative to said drive-wheel; a spring; power means coupled tosaid spring for adjusting the bias or spring force of said spring means,said power means including means responsive to change in weight of aload carried by said load carrying device for increasing said bias orspring force exerted on said drive-wheel and thus said braking force ofthe industrial truck with increasing weight of the load carried by saidload carrying device.
 2. An arrangement according to claim 1,wherein:said load carrying device includes a hydraulic systemoperatively coupled to said hydraulic lifting arrangement; and saidpower means includes a working piston-cylinder device which is connectedto said hydraulic systemm of said load carrying device such that aload-dependent working pressure in said hydraulic system can betransmitted to said piston-cylinder device to influence said bias orspring force.
 3. An arrangement according to claim 3, wherein saidspring means includes a coil spring placed around said piston-cylinderdevice.
 4. An arrangement according to claim 1, wherein said industrialtruck includes a power assembly for said drive-wheel, said drive-wheeland its power assembly comprising a unit which is sprung relative tosaid chassis.
 5. An arrangement according to claim 4, wherein saiddrive-wheel has a wheel-hub, and said power assembly includes anelectric motor located above said drive-wheel and a gear unit located atsaid wheel-hub.
 6. An arrangement according to claim 5, wherein saidpower assembly is carried by a motor support and said coupling meansincludes at least two link arms which are arranged in parallel betweensaid chassis and said motor support and attached thereto.
 7. Anarrangement according to claim 6, wherein said spring means includesanother spring element arranged between said chassis and said motorsupport.
 8. An arrangement according to claim 7, wherein said electricmotor is arranged vertically with its rotation axis arrangedperpendicular to the drive-wheel axis, and said power assembly isjournalled in relation to said motor support by means of a bearing. 9.An arrangement according to claim 1, further comprising control meanscooperatively coupled to said hydraulic lifting arrangement fordisconnecting said spring-force increasing power means when said loadcarrying device lifts a load thereon above a given horizontal level. 10.In an industrial truck, an arrangement for increasing the braking forceof at least one wheel of the industrial truck relative to a drivingsurface on which the industrial truck is driven, the industrial truckincluding a chassis carried on support wheels which have substantiallyno spring suspension and which are driven and braked by a sprungdrive-wheel arranged between two of the support wheels and a loadcarrying device which can be raised and lowered relative to the chassisby means of a hydraulic lifting arrangement, and which truck furtherincludes a spring means arranged between the chassis and the sprungdrive-wheel and which is adapted to exert on said drive-wheel a springforce acting towards said driving surface,the improvement wherein saidspring means includes: biasing means arranged between said chassis andsaid drive-wheel, said biasing means producing a bias or spring force;power means coupled to said biasing means for adjusting the bias orspring force of said biasing means, said power means including meansresponsive to change in weight of a load carried by said load carryingdevice for increasing said bias or spring force exerted on saiddrive-wheel and thus said braking force of the industrial truck withincreasing weight of the load carried by said load carrying device. 11.An arrangement according to claim 10, wherein:said load carrying deviceincludes a hydraulic system operatively coupled to said hydrauliclifting arrangement; and said power means includes a workingpiston-cylinder device which is connected to said hydraulic system ofsaid load carrying device such that a load-dependent working pressure insaid hydraulic system can be transmitted to said piston-cylinder deviceto influence said bias or spring force.
 12. An arrangement according toclaim 11, wherein said biasing means includes a coil spring placedaround said piston-cylinder device.
 13. An arrangement according toclaim 10, wherein said spring means includes at least two link armsarranged between said chassis and said drive-wheel.
 14. An arrangementaccording to claim 13, wherein said biasing means is arranged betweenone of said at least two link arms and said chassis.
 15. An arrangementaccording to claim 10, wherein said industrial truck includes a powerassembly for said drive-wheel, said drive-wheel and its power assemblycomprising a unit which is sprung relative to said chassis.
 16. Anarrangement according to claim 15, wherein said drive-wheel has awheel-hub, and said power assembly includes an electric motor locatedabove said drive-wheel and a gear unit located at said wheel-hub.
 17. Anarrangement according to claim 16, wherein said power assembly iscarried by a motor support and said spring means includes at least twolink arms which are arranged in parallel between said chassis and saidmotor support and attached thereto.
 18. An arrangement according toclaim 17, wherein said spring means includes another spring elementarranged between said chassis and said motor support.
 19. An arrangementaccording to claim 18, wherein said electric motor is arrangedvertically with its rotation axis arranged perpendicular to thedrive-wheel axis, and said power assembly is journalled in relation tosaid motor support by means of a bearing.
 20. An arrangement accordingto claim 10, further comprising control means cooperatively coupled tosaid hydraulic lifting arrangement for disconnecting said spring-forceincreasing power means when said load carrying device lifts a loadthereon above a given horizontal level.